1,3-butylene glycol is an organic compound which has a boiling point of 208.degree. C. under ordinary pressure, is viscous, colorless, transparent and low odor, exhibits an excellent solubility and a capability of producing chemically-stable derivatives, and is a useful compound as a solvent for coatings, starting materials for various synthetic resins and surfactants, a high-boiling-point solvent and antifreeze, food supplements, animal food supplements, a humectant for tobacco composition and an intermediate for preparation of various other compounds.
Recently in particular, high-quality odorless 1,3-butylene glycol has been used as a solvent for toiletry products in the field of cosmetics due to its excellent moisture absorptive property, low volatility, low irritation and low toxicity.
However, the scope for the application of 1,3-butylene glycol is limited due to a very minor quantity of residual odor.
Most recently in particular, further improvement in the quality and the yield of an odorless, so-called "cosmetic grade" 1,3-butylene glycol has been strongly desired.
Heretofore, there have been three widely-known processes for the preparation of 1,3-butylene glycol which are described below.
(1) A method (British Patent N.sup.o 853266) in which acetaldol is first prepared by aldol condensation of acetaldehyde, and then catalytically hydrogenated to obtain 1,3-butylene glycol. PA0 (2) A method in which 1,3-butylene glycol is prepared by a hydration reaction of 1,3-butyleneoxide. PA0 (3) A method in which 1,3-butylene glycol is prepared from propylene and formaldehyde by the Prince Reaction. PA0 (a) aldol condensation of acetaldehydc in the presence of an alkali catalyst to obtain a reaction crude solution primarily containing aldoxane, acetaldehyde and water; PA0 (b) thermal decomposition of aldoxane to obtain paraldol while distilling off a distillate containing acetaldehyde, water and small amounts of croton aldehyde from the reaction crude solution; PA0 (c) hydrogenation of paraldol to obtain 1,3-butylene glycol in the presence of a catalyst;
However, method (2) is not yet completed for use as an industrial manufacturing process, and therefore is unpractical.
And, method (3) is also unpractical because it gives only a low yield.
Therefore, 1,3-butylene glycol has been industrially manufactured by method (1).
However, acetaldol is not stable due to its chemical structure.
For example, croton aldehyde is produced by dehydration of acetaldol, resulting in the production of various impure materials as by-products, for example, butanol, 2-butanone, etc.
It is known that the above-mentioned acetaldol produced industrially is primarily composed of 2,4-dimethyl-1,3-dioxane-6-ol(aldoxane) which is a trimer of acetaldehyde as described in Industrial Engineering Chemistry 44, 1003 (1952).
It is self-evident that if aldoxane is catalytically reduced, it would be decomposed by hydrogenation into 1,3-butylene glycol and ethanol as a reaction mechanism, which makes it objectionable for the purpose of industrially manufacturing 1,3-butylene glycol.
As a method for solving the problem, for example, Japanese Patent Unexamined Publications 212384/1987 and 246529/1987 disclose that a crude reaction solution primarily consisting of paraldol is prepared while distilling out acetaldehydc after thermally decomposing aldoxane, followed by catalytically reducing paraldol to prepare 1,3-butylene glycol.
Heretofore, unreacted acetaldehyde [a primary component in stream (C) of FIG. 1] distilled out in a step [aldoxane decomposing column 1-3 in FIG. 1] for preparing a solution [stream (B) of FIG. 1] primarily consisting of aldoxane and paraldol in which aldoxane [a primary component in stream (A) of FIG. 1] is thermally decomposed, has been recirculated [stream line (F)] to the aldol condensation step (a) [1-1 of FIG. 1].
In such an acetaldehydc-recirculating method, croton aldehyde generated in the thermal decomposition step (b) [1-3 of FIG. 1] of aldoxane is also recirculated to the aldol condensation step together with unreacted acetaldehyde, unpreferably resulting in the generation of various impure components by a reaction with acetaldehyde, etc. in the aldot condensation step.
The impure components, in particular, odor-causing impurities cannot be sufficiently removed in succeeding steps [e.g. ethanol/butanol distillation columns, water distillation column, etc.], resulting in it adversely affecting to the quality (e.g. odor regulation) of 1,3-butylene glycol products for their uses in the field of cosmetics.
Almost all of the water is removed together with the unreacted acetaldehyde through the thermal decomposition step of aldoxane and the simultaneous acetaldehyde distillation as described above, objectionably resulting in it becoming unsuitable of practical operation due to viscosity increase or crystallization of the crude solution discharged from the bottom of the decomposition column.
Furthermore, conventional 1,3-butylene glycol products prepared by the hydrogenation of paraldol after removing croton aldehyde as described above include minor amounts of odor-causing impurities even after being refined through the conventional refining steps described below.
The conventional refining steps primarily have included an ethanol distillation step, a butanol distillation step, a water distillation step, a salts-removing step, a step for removing high-boiling-point ingredients and a step for removing low-boiling-point ingredients, leading to the production of a refined 1,3-butylene glycol.
On the other hand, it is known (e.g. Japanese Examined Patent Publication N.sup.o 80139/1991) that very minor amounts of odor-causing impurities can be acceleratedly removed by distillation while charging water so as to obtain an odorless 1,3-butylene glycol (e.g. a product having a purity of 99.7-99.97%) from a 1,3-butylene glycol product having odor (e.g. a commercially supplied product having a purity of more than 98%).
Even though the method described in the Publication n.sup.o 80139/1991 is carried out, the odorless 1,3-butylene glycol an be obtained only at a yield of 50 to 60%.
The present invention has been completed as a result of studies by the present inventor to solve the above-described problems.